The present invention relates to certain novel pyrimidine intermediates, processes for their preparation and their conversion to 9-substituted-2-aminopurines, such as certain carboxylic, heterocyclic and acyclic purine nucleoside analogues, and salts, esters and pharmaceutically acceptable derivatives thereof.
A number of 2-aminopurine nucleoside analogues have been shown to be useful in the treatment or prophylaxis of viral infections, for example the compound of formula (A) 
is described as having potent activity against human immunodeficiency virus (HIV) and hepatitis B virus (HBV) (EP 0434450).
Processes have been proposed for the preparation of 9-substituted-2-aminopurines, generally starting from a pyrimidine compound, coupling with a sugar analogue residue, and cyclisation to form the imidazole ring and introduction of any suitable 6-substituent.
Pyrimidine compounds which have been identified as being useful in the preparation of 9-substituted-2-aminopurines include 2,5-diamino-4,6-dichloropyrimidine, N,Nxe2x80x2-(4,6-dichloro-2,5-pyrimidinediyl)bis formamide and also N-2-acylated pyrimidine derivatives such as the 2-acetamido and 2-isobutryamide derivatives (U.S. Pat. No. 5,087,697).
Processes for the synthesis of these intermediates generally involve a number of steps of which some are difficult to perform and produce poor yields, preventing any practical scale up of these processes above a few grams, and are thus difficult and uneconomical.
Processes for the synthesis of the intermediate 2,5-diamino-4,6-dichloropyrimidine include the direct chlorination of readily available 2,5-diamino-4,6-dihydroxypyrimidine using phosphorus oxychloride. The original examination of this reaction was carried out by Temple et al. (J. Org. Chem. 1975, 40: 3141-3142). These workers concluded that the reaction was unsuccessful, apparently because of degradation of the pyrimidine ring system. Hanson (SmithKline Beecham, WO 91/01310, U.S. Pat. No. 5,216,161) subsequently described a process for the direct chlorination of 2,5-diamino-4,6-dihydroxypyrimidine by refluxing with phosphorus oxychloride in the presence of large molar excesses of quaternary ammonium chlorides or amine hydrochlorides. We have examined this process and have obtained, repeatedly, much lower yields ( less than 10%) of crude 2,5-diamino-4,6-dichloropyrimidine than those specified in the SmithKline Beecham patent specification. The extensive decomposition of the 2,5-diamino-4,6-dihydroxypyrimidine to tars which coat the equipment, combined with the problems of dealing with the copious solids due to the insoluble amine salts, constitute significant drawbacks and make scale-up of such a process impractical. The modifications of Legraverend et al. (Synthesis 1990: 587-589), namely using acetonitrile as a solvent and adding phosphorus pentachloride to the phosphorous oxychloride and quaternary ammonium chloride, result, in our experience in the isolation of approximately 30% (after chromatographic purification) of 2,5-diamino-4,6-chloropyrimidine on a 2-5 gram scale. Again, scale-up beyond a few grams is impractical due to the formation of tarry precipitates.
A recent Lonza AG patent specification (EP 0 552 758) suggests that higher yields (35-65%) may be obtained with phosphorus oxychloride chlorination when the 5-amino group of 2,5-diamino-4,6-dihydroxypyrimidine is protected with an alkoxycarbonyl protecting group. This modification is claimed to simplify the chlorination step in that the amines and phosphorus pentachloride, employed in the prior processes discussed above are not required. This creates a new problem, namely the need to remove the alkoxycarbonyl protecting groups in order to be able to convert the pyrimidine intermediates to purines. Indeed, the Lonza AG specification does not show that such 5-protected 2,5-diamino-4,6-dichloropyrimidines may be converted to purines in an advantageous manner.
A process for the synthesis of N,Nxe2x80x2-(4,6-dichloro-2,5-pyrimidinediyl)bis formamide is the reaction of 2,5-diamino-4,6-dichloropyrimidine with formic acid and acetic anhydride (Harden et al., J. Med. Chem. 1990, 33:187-196 and U.S. Pat. No. 5,159,076).
The 5-step route to the N-2-acylated derivatives, and also to 2,5-diamino-4,6-dichloropyrimidine required for the synthesis of N,Nxe2x80x2-(4,6-dichloro-2,5-pyrimidinediyl)bis-formamide (Temple et al., J. Org. Chem. 1975, 40: 3141-3142), starts from 2-amino-6-chloropyrimidine-4-one and contains steps, which include the introduction of the 5-nitro group and the subsequent handling and reduction of very reactive 5-nitro-4,6-dichloropyrimidine intermediates, which make scale-up impractical. The yields on a number of the steps to these intermediates are poor (Legraverend et al., Synthesis 1990: 587-589).
We have now discovered certain new pyrimidine intermediates which are useful in a new synthetic route for the preparation of the above 9-substituted-2-aminopurines and in addition which can be used in the synthesis of the known intermediates described above.
In one aspect of this invention we provide the following novel intermediates which may be utilised in the synthesis of 2-aminopurines, namely compounds of formulae (I), (II) and (III); 
wherein R1 and R2, which be the same or different, are selected from C1-8 straight-chain alkyl, C1-8 branched alkyl C3-8 cycloalkyl, and aryl groups (such as phenyl or naphthyl), which may be optionally substituted, for example by C1-4 alkyl or halogen (e.g. Cl). In a preferred embodiment of the invention R1 and R2 are both methyl.
These novel intermediates can be readily prepared in good yields and are useful for the preparation of a wide variety of different types of 2-aminopurines including the nucleoside analogue of formula (A), famciclovir (EP 0182024), penciclovir (EP 0141927), H2G (EP 0343133), (1xe2x80x2S,3xe2x80x2S,4xe2x80x2S)-2-amino-1,9-dihydro-9-[3,4-dihydroxy-3-hydroxymethyl-1-cyclopentyl]-6H-purin-6-one (EP 0420518), and other 9-substituted-2-aminopurines provided that the 9-substituent is not attached by a glycosidic bond.
In a further aspect of this invention we provide processes for the synthesis of the novel intermediates of formulae (I), (II) and (III), and the known intermediate 2,5-diamino-4,6-dichloropyrimidine(IV). These processes are illustrated in the simplified diagram below which is designed for illustration only of the possible ways of synthesising these intermediates; 
The present invention also provides a process for the preparation of compounds of formula (I) which comprises chlorination of 2,5-diamino-4,6-dihydroxypyrimidine with a halomethylenimminium salt (Vilsmeier reagent) of formula (V). 
wherein R1 and R2 are as defined above.
Compounds of formula (V), may be prepared from a variety of formamides of secondary amines by reaction with a variety of acid halides, such as phosphorus oxychloride, phosphorus pentachloride, thionyl chloride, phosgene, and oxalyl chloride, for example as detailed in a review by C. M. Marson, Tetrahedron 1992, 48: 3660-3720 and references therein.
The advantage of protecting the diaminopyridine from extensive decomposition during chlorination is achieved by the in situ protection of the amino groups with two molar equivalents of Vilsmeier reagent (V) to give a bisformamidine intermediate (detected by thin-layer chromatography), which is subsequently chlorinated to a compound of formula (I) as the reaction with additional equivalents of Vilsmeier reagent proceeds. The improved solubility of such bis-formamidine derivatives is an added advantage of this process, facilitating the subsequent chlorination to compounds of formula (I) and their isolation and simple purification.
The disadvantage of the use of 5-alkoxycarbonyl protecting groups, as described in the Lonza specification (EP 0552758) is avoided since the formamidine groups in compounds of formula (I) are readily hydrolysed under mild conditions in a step-wise manner to form the intermediates (II) and (III); or alternatively compounds of formula (I) can be directly hydrolysed to compounds of formula (III).
The compound 2,5-diamino-4,6-dichloropyrimidine (IV) can be prepared by:
A) the hydrolysis of a compound of formula (I),
B) the hydrolysis of a compound of formula (II); or
C) the hydrolysis of a compound of formula (III).
The hydrolysis of (I), (II), or (III) to 2,5-diamino-4,6-dichloropyrimidine is conveniently carried out at pH 3+/xe2x88x920.5 by adding a water-miscible cosolvent, such as ethanol. The hydrolysis is more efficient at pH 1-2, with shorter reaction times required than at a higher pH. It is advisable at pH 1-2, however, to protect 2,5-diamino-4,6-dichloropyrimidine from hydrolysis to hydroxypyrimidines by exaction, as it is formed, into an organic layer which is not miscible with the aqueous acid. When the pH of the aqueous layer is below 1, extraction of the product into the organic layer is inefficient (the pKa of (IV) was found to be ca 0.5 and the pyrimidine ring is thus significantly protonated below pH 1). Preferably, the acid used for this hydrolysis should be one which is not appreciably soluble in the organic layer, e.g. phosphoric or sulfic acid. The organic solvent should be one which is stable to aqueous acid and in which (IV) is soluble. Satisfactory solvents for the organic layer include toluene and halocarbon solvents such as methylene chloride, chloroform, and 1,2-dichloroethane. At completion, the organic layer is simply washed, e.g. with saturated aqueous bicarbonate, dried and concentrated to provide (IV) with no purification required.
Compounds of formula (III) can be prepared by:
A) selective hydrolysis of a compound of formula (I); or
B) selective hydrolysis of a compound of formula (II).
The hydrolysis of compounds of formula (I) or (I) to (III) is most efficiently carried out in dilute aqueous acid, preferably in dilute aqueous mineral acid such as sulfuric acid, hydrochloric acid, or phosphoric acid. Prolonged exposure to pH below 1 should be avoided as the chloropyrimidine ring is protonated significantly below pH 1 and may therefore undergo attack by water, generating undesired hydroxypyrimidine by-products. Preferably, the pH is maintained above 2 and optimally at 3+/xe2x88x920.5 for the efficient formation of (III). In this optimal pH range, the formamidine groups of (I) and (II) are selectively hydrolysed to give (III) in approximately 70% yield. As the hydrolysis of the formamidine groups of (I) and (II) proceed, the secondary amine from which the Vilsmeier reagent (V) was formed (HNR1R2) is liberated and causes the pH of the solution to rise, thus slowing the hydrolyses. In addition, with certain reactive aliphatic amines HNR1R2, such as N,N-dimethylamine, it is necessary to maintain a pH sufficiently low to prevent the chloro groups of the pyrimidine ring from displacement by the secondary amine. We have found that maintaining the pH of the reaction mixtures below 4 avoids significant displacement of the chloro groups by the secondary amine, even with amines as reactive as N,N-dimethylamine. It was thus found optimal to buffer the hydrolyses of (I) and (II) to (III) at pH 3+/xe2x88x920.5 or to add increments of acid throughout the hydrolyses in order to maintain the pH in this range.
Optimally, the hydrolysis of compounds of formula (I) or (II) to (III) is carried out in a minimum of water with the pH controlled as described above. Under these conditions, (III) precipitates as formed and is simply filtered off and washed with water. The hydrolysis is carried out at gentle reflux for 4 hours, or at lower temperatures for longer periods of time.
The compounds of formula (II) can be prepared by the selective hydrolysis of the compounds of formula (I). Preferably the selective hydrolysis is carried out with slightly more than two molar equivalents of mineral acid in water or ethanol and warmed for 15-30 minutes.
The compounds of formula (I) can be prepared by reacting 2,5-diamino-4,6-dihydroxypyrimidine with a Vilsmeier reagent of formula (V).
The compound 2,5-diamino-4,6-dihydroxypyrimidine is commercially available (Sigma, Maybridge BTB, Pfaltz and Bauer, Polyorganix).
The novel bis-formamidines of formula (I) are formed and isolated conveniently in high yield when the 2,5-diamino-4,6-dihydroxypyrimidine (or a salt thereof such as the hydrochloride or the hemisulfate) is treated with at least 4 molar equivalents of a Vilsmeier reagent (V). These chlorination reactions proceed under extremely mild conditions without the formation of copious tarry precipitates which characterises direct chlorinations, as previously described with phosphorus oxychloride and phosphorus oxychloride/quaternary ammonium halides. The Vilsmeier chlorination of 2,5-diamino-4,6-dihydroxypyrimidine may be carried out in an inert solvent, such as toluene, chloroalkenes, or chloroalkanes (such as methylene chloride, chloroform or 1,2-dichloroethane). Preferably the solvent is 1,2-dichloroethane, chloroform, or methylene chloride. The chlorination may be carried out at 0 to 110xc2x0 C., preferably at 40-100xc2x0 C., conveniently at reflux for the solvent used. Reaction times are typically 12 to 48 hours. Isolation of compounds of formula (I) is simple and can be readily scaled-up, involving simply washing the reaction solution with an aqueous solution containing sufficient base, such as sodium bicarbonate, to neutralize any hydrogen chloride formed and then concentrating the dried organic layer to obtain the novel chlorinated pyrimidines of formula (I). The compounds of formula (I) are generally stable and may be precipitated from a variety of solvents, such as ethyl acetate, and stored or used without further purification.
Particularly preferred examples of the compounds of formulae (I), (II) and (III) are:
a) 4,6-Dichloro-2,5-bis-[(dimethylamino)methyleneamino]pyrimidine
b) 2-Amino-4,6-dichloro-5-[(dimethylamino)methyleneamino]pyrimidine
c) N-(2-Amino-4,6-dichloro-5-pyrimidinyl)formamide
According to a further aspect of this invention the novel intermediate of formula (III) can be used in the synthesis of 2-amino-6-chloropurines. In addition compounds of formula (I) or (II) may also be used in the synthesis of 2-amino-6-chloropurine nucleosides, provided that the amine HNR1R2 (where R1 and R2 are defined earlier) liberated, during the conversion of the pyrimidine intermediate to the purine, is sufficiently unreactive towards the displacement of the chloro group of the 2-amino-6-chloropurines generated.
The compounds of formula (III) share with the previously described N-2-acylated derivatives the property of greater than 2,5-diamino-4,6-dichloropyrimidine toward displacement of a chloro group by an appropriate primary amine or protected hydroxylamine. However, such condensations with (III) (described in more detail below) may be carried out under milder conditions at lower temperatures and with shorter reaction times than with compound (IV), thus resulting in less decomposition of the amines. The condensation products (VI) are isolated in greater yield and purity than the corresponding products (VIII) formed in condensations with 2,5-diamino-4,6-dichloropyrimidine (IV). Another advantage of the use of the intermediate (III) over the previously described N-2-acylated derivatives, in addition to greater ease of synthesis, is that the purines generated from (III) do not require deprotection, i.e. hydrolysis of the N-2-acyl group (these longer processes are described in U.S. Pat. Nos. 5,087,697 and 5,159,076). 
Wherein R3 is hereinafter defined.
The compound of formula (III) can be used to prepare the novel intermediates of formula (VI) which represent a further feature of the invention: 
wherein R3 may be hydrogen or any group which is not attached by a glycosidic bond.
Preferably R3 is a hydroxyl or a protected hydroxyl; or a carbocyclic croup (e.g. C3-7 carbocyclic), an acylic group (e.g. C2-8 hydrocarbyl) wherein carbon atoms may be substituted by one or more heteroatoms such as N, O or S, or a heterocyclic group (e.g. C4-7 heterocyclic) in which at least one carbon atom is replaced by a N, O or S atom, or a substituted analogue of any thereof (e.g. such substituents are independently selected from C1-4alkyl, C1-4 alkoxy, hydroxyl or protected hydroxyl azido, phosphonyl or halogen), provided that such groups are not attached by a glycosidic bond.
Preferred groups for R3 are hydroxyl or protected hydroxyl.
Further preferred groups for R3 are 
A further preferred group for R3 is; 
Suitable groups for R3 are selected from a; b; c; d; e; and f, as defined above.
By xe2x80x9chydrocarbylxe2x80x9d it is meant a group containing only hydrogen and carbon atoms, which may contain double and/or triple bonds and which may be straight, branched, cyclic or aromatic.
According to a further feature of the invention we provide a process for the preparation of compounds of formula (VI) which comprises reacting a compound of formula (III) with an amine of formula R3NH2, where R3 is defined above. Such condensations are preferably carried out at reflux in a solvent such as ethanol, butanol, water or acetonitrile in the presence of at least one equivalent of a base, such as trialkylamine or potassium or sodium carbonate.
Subsequent references to compounds of formula (VIa, b, c, d, e, f, g, or h) denote a compound of formula (VI) in which R3 is a group of a, b, c, d, e, f, g, or h as defined above.
A particularly preferred compound of formula (VI) is (1S,4R)-4-[(2-amino-6-chloro-5-formamido-4-pyrimidinyl)amino]-2-cyclopentene-1-methanol (VIa).
The novel intermediates (VI) can be converted by ring closure to the corresponding compounds of formula (VII): 
wherein R3 is defined above.
Ring closure of (VI) to (VII) is conveniently carried out in trialkylorthoformates (e.g. triethylorthoformate or timethylorthoformate) with concentrated aqueous acid (e.g. 2-4 molar equivalents of hydrochloric, sulfuric acid or methane sulfonic). For example, the hydrochloride salt of (VIIa) i.e. wherein R3 represents group a, begins to precipitate from such orthoformate solutions of (VIa) within minutes and yields above 90% may be achieved by filtration of the precipitate, optimally after several hours at ambient temperature.
The synthesis of 9substituted-2-amino-6-chloropurines, such as compounds of formula (VII), in this manner represents a significant improvement over previously published syntheses utilizing pyrimidine Mediates such as (VIII): 
as described U.S. Pat. No. 4,916,224. The previously-described routes to intermediates such as (VIII) are longer and, more importantly, the number of steps to the purine targets after incorporation of the group R3 is greater. Also, triaminopyridine intermediates such as (VIII) are air- and light-sensitive and extremely difficult to purify due to their polarity and metal-chelating abilities (the isolation from the zinc reduction of diazo intermediates is especially problematic). The novel 5-formamido intermediates of formula (VI) are easily and directly attainable from compounds of formula (III) in one step and are generally solids which are stable and easily-purified by precipitation from a suitable solvent.
(1xe2x80x2S,3xe2x80x2S,4xe2x80x2S)-2-Amino-1,9-dihydro-9-[3,4-dihydroxy-3-hydroxymethyl-1-cyclopentyl]-6H-purine-6-one (IXh) (EP0420,518) may be prepared by condensation of the compound of formula (III) with 4-amino-3-cyclopentene-1-methanol (U.S. Pat. No. 5,049,671) to form the compound of formula (VIg) followed by ring closure of the compound of formula (VIg) to prepare the compound of formula (VIIg), which may be hydroxylated, with osmium tetroxide/N-methyl-morpholine N-oxide to provide the compound of formula (VIIh). The compound of formula (VIIh) is hydrolysed to form the compound of formula (IXh).
2-Amino-6-chloropurine (VIIb) may be prepared by ring closure of novel 2,4-diamino-6-chloro-5-formamidopyrimidine (VIb), conveniently synthesized by condensation of the compound of formula (III) with ammonia. The compound of formula (VIIb) is an intermediate suitable for the synthesis of acyclic antiviral nucleosides, such as famciclovir wherein the 2-amino-6-chloropurine intermediate (VIId) is conveniently subjected to hydrogenolysis to the 2-aminopurine nucleoside.
Carbocyclic nucleosides may also be synthesized from the compound of formula (VIIb), for example by (Pd-catalyzed coupling with an appropriate carbocyclic intermediate as described in Mac Keith et al., J. Chem. Soc. Perkin Trans 1. 1993: 313-314 and references therein.
The compounds of formula (VIIa), (VIIc), (VIIe), (VIIf), (VIIg) and (VIIh) are conveniently hydrolyzed to the corresponding guanine compound by refluxing with aqueous base or acid.
As a further feature of this invention we have found an alternative process for the synthesis of 2,6-diaminopurine (wherein the 6-amino group is substituted by R4 and R5, which may be the same or different, and are selected from H, C1-8alkyl, C3-6cycloalkyl, aryl (such as phenyl), in particular R4 is H and R5 is cyclopropyl) directly from (VI) which advantageously eliminates a step in the process. Such 2-aminopurine compounds can be synthesised directly from the intermediates (VI) by refluxing the compound of formula (VI) with an excess of the amine (HNR4R5) in a refluxing solvent, such as ethanol, isopropanol, n-propanol, t-butanol or n-butanol.
In particular cases, it may be more useful to utilize 2,5-diamino-4,6-dichloropyrimidine(IV) to prepare compounds of formula (VIII), useful in the synthesis of 8-modified 2-aminopurine nucleoside analogues, such as 8-aza-2-aminopurines (which have broad-spectrum anti-herpes activities described in Storer et al., Spec. Publ. Roy. Soc. Chem (Rec. Adv. Chem. Anti-Infect. Agents) 1993, 119: 251-265); in such cases the intermediates (I), (II) and (III) can be used to provide (IV).
Pharmaceutically acceptable esters of certain compounds of the invention may be prepared by esterification using conventional methods known in the art. Such methods include, for example, the use of an appropriate acid halide or anhydride.
The compounds of the invention, including esters thereof may be converted into pharmaceutically acceptable salts in a conventional manner by treatment with an appropriate acid or base. An ester or salt of a compound of the invention may be converted into the parent compound, for example, by hydrolysis.
The following examples are intended for illustration only and are not intended to limit the scope of the invention in any way.